1. Field of the Invention
This invention relates generally to static VAR generators and more particularly to static VAR generators having circuit arrangements to reduce harmonics generated by thyristor-controlled inductors.
2. Description of the Prior Art
In a conventional static VAR generator, a thyristor-controlled inductor is employed as a means to provide continuously variable reactive power output. The reactive power control is achieved by the variation of the firing delay angle .alpha. of the thyristor switch, which determines the current flow in the inductor. This method of reactive power control inherently generates harmonics, the magnitude of which are a function of the firing delay angle. Various circuit configurations and control methods have been proposed to reduce the magnitudes of harmonics generated by the thyristor-controlled inductor.
One method employs a 12-pulse circuit arrangement in which two three-phase inductor banks are operated from two three-phase voltage sets that are phase displaced by 30 electrical degrees. The sets are normally provided by a coupling transformer with appropriate secondary windings. In the 12-pulse circuit arrangement, some harmonics such as the fifth and seventh, cancel each other in the primary of the transformer. The disadvantage of this method of harmonic reduction is that it relies on a perfectly balanced circuit and power system, a condition which is difficult to achieve in practice. Also, this method does not reduce some significant harmonics such as the 11th and 13th.
There is shown in U.S. Pat. No. 4,104,576, issued to Harry Frank on Aug. 1, 1978 a static VAR generator system that employs a number of parallel-connected inductors, of which only one has a firing delay angle control, the others are fully on or off for a multibank circuit arrangement. In this static VAR generated design, each harmonic is reduced by the number of inductor banks employed. The disadvantage of this method is that, at a given operating voltage and output power rating established to optimize other components in the static VAR generator or the power system, the thyristor switches may not be fully utilized for their rating which greatly increases the cost of the overall system. This is because the economic utilization of the thyristor switches requires that the full current conducted be close to the rated current of the switches. In the static VAR genrator disclosed, generally a number of inductors, n, is operated at the same voltage level to provide a given output power, the current in each thyristor switch is 1/n of that which would flow if a single inductor was used to provide the same output power. If the output rating required is such that one thyristor switch, at the given voltage level could handle the total current which is often the case in practice, then the harmonic reduction can only be achieved at a considerable cost.